Our transcription: Petroleum exploration requires locating the geologic structures that trap oil and gas.These structures are usually marked by changes in density underground.Geologists are able to detect these changes by bouncing seismic waves through Earth's crust.And you have to keep in mind that down in the subsurface of the Earth, the rocks have different hardnesses.And when you have two formations or two rock layers that have a different hardness, and they're in juxtaposition or in contact with each other, then you can get a reflection back.Just like looking into a mirror you get a reflection of your face.Just like the sonogram reflects off of the baby's body and returns its image to the surface.